Controllable seismic source

ABSTRACT

An apparatus for generating seismic waves includes a housing, a strike surface within the housing, and a hammer movably disposed within the housing. An actuator induces a striking motion in the hammer such that the hammer impacts the strike surface as part of the striking motion. The actuator is selectively adjustable to change characteristics of the striking motion and characteristics of seismic waves generated by the impact. The hammer may be modified to change the physical characteristics of the hammer, thereby changing characteristics of seismic waves generated by the hammer. The hammer may be disposed within a removable shock cavity, and the apparatus may include two hammers and two shock cavities positioned symmetrically about a center of the apparatus.

GOVERNMENT INTERESTS

The present invention was developed with support from the U.S.government under a contract with the United States Department of Energy.Contract No. DE-NA0000622. Accordingly, the U.S. government has certainrights in the present invention.

FIELD

Embodiments of the present invention relate to seismic sources for usein seismic surveying applications. More particularly, embodiments of thepresent invention relate to seismic sources with selectively controlledoperation configured to generate seismic waves with differentcharacteristics.

BACKGROUND

Seismic surveying is a technique for generating information about anarea below the earth's surface to identify subterranean structuralfeatures, such as voids or changes in composition. Petroleum companies,for example, use seismic surveying to explore for oil and gas reserves.Seismic surveying involves introducing one or more seismic waves intothe area to be surveyed and sensing seismic activity at one or morelocations on and/or below the surface of the earth in or near thesurveyed area. The seismic waves may be generated, for example, by acontrolled explosion or large “hammer” strike at the surface of the areato be imaged. One or more seismic sensors may be placed on and/or belowthe earth's surface to detect seismic activity caused by the waves. Suchseismic activity typically includes seismic energy reflected back to theearth's surface as the seismic waves encounter structuraldiscontinuities in the surveyed area. Seismic surveying systems use theseismic activity information collected by the sensors to generateinformation about subterranean composition and structure and may expressthe information in the form of paper traces or display images.

Seismic waves may be classified as compressional waves or shear waves.Compressional waves, sometimes referred to as primary waves, pressurewaves or P-waves, are generally longitudinal in that the particles inthe medium through which the waves travel vibrate along or parallel tothe direction of travel of the wave energy. Shear waves, also referredto as secondary waves or S-waves, are generally transverse in that theparticles in the medium through which the waves travel vibrate in adirection that is perpendicular or transverse to the direction of travelof the wave energy. Compressional waves and shear waves possessdifferent characteristics. Compressional waves, for example, typicallypropagate at a higher speed than shear waves and are capable ofpropagating through fluid, while shear waves are not.

The above section provides background information related to the presentdisclosure which is not necessarily prior art.

SUMMARY

An apparatus for generating seismic waves constructed in accordance withan embodiment of the present invention comprises a housing, a strikesurface within the housing, and a hammer movably disposed within thehousing. An actuator induces a striking motion in the hammer wherein thehammer impacts the strike surface as part of the striking motion. Theactuator is selectively adjustable to change characteristics of thestriking motion and characteristics of seismic waves generated by theimpact. During operation, the apparatus is placed against a surface ofthe ground such that when the hammer impacts the strike surface energyis transferred from the hammer to the housing and from the housing intothe ground to generate seismic waves. The strike surface may be normalto the surface of the ground so that the apparatus primarily orexclusively generates shear seismic waves.

An apparatus constructed in accordance with a related embodiment of theinvention includes a first shock structure removably disposed within thehousing and defining a first shock cavity, a first hammer movablydisposed within the first shock structure, a second shock structureremovably disposed within the housing and defining a second shockcavity, and a second hammer movably disposed within the second shockstructure. The actuator is operable to move the hammers and cause eachof the hammers to strike an internal surface with a striking motion. Theactuator is selectively adjustable to modify characteristics of thestriking motion of each of the hammers, including hammer speed andacceleration.

In another embodiment of the invention, a method of generating seismicwaves comprises placing an apparatus against a ground surface. Theapparatus is configured to generate seismic waves and includes a shockcavity, an actuator, and a first hammer positioned within the shockcavity. The actuator is controlled to induce a first striking motion inthe first hammer such that the first hammer strikes a wall of the shockcavity and generates a first seismic wave. The actuator is alsocontrolled to induce a second striking motion in the first hammer suchthat the first hammer strikes the wall of the shock cavity and generatesa second seismic wave, wherein the second seismic wave is different thanthe first seismic wave.

An apparatus for generating seismic waves constructed in accordance withanother embodiment of the invention comprises a housing, a hammermovably disposed within the housing, and an actuator for inducingcontinuous oscillating motion in the hammer, wherein the oscillatingmotion of the hammer creates a seismic wave. The actuator is selectivelyadjustable to change characteristics of the oscillating motion andcharacteristics of seismic waves generated by the motion.

This summary is provided to introduce a selection of concepts in asimplified form that are further described in the detailed descriptionbelow. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Other aspectsand advantages of the present invention will be apparent from thefollowing detailed description of the embodiments and the accompanyingdrawing figures.

DRAWINGS

Embodiments of the present invention are described in detail below withreference to the attached drawing figures, wherein:

FIG. 1 is a rear elevation view of a seismic source apparatusconstructed in accordance with an embodiment of the invention.

FIG. 2 is a rear perspective view of a vehicle for mounting theapparatus of FIG. 1 and including a plurality of seismic sensorsattached to the vehicle.

FIG. 3 is a top perspective exploded view of a portion of the seismicsource apparatus of FIG. 1.

FIG. 4 is a top perspective exploded view of a shock assembly of theapparatus of FIG. 1.

FIG. 5 is a schematic diagram of a seismic survey system with which theapparatus of FIG. 1 may be used.

FIG. 6 is a schematic diagram of a controller of the apparatus of FIG.1.

FIG. 7 is a graph of an exemplary seismic frequency response of a manualseismic source using a five pound sledge hammer.

FIG. 8 is a graph of an exemplary seismic frequency response of theseismic source of the present invention.

FIG. 9 is a graph of an exemplary seismic frequency response of a manualseismic source using a ten pound sledge hammer.

The drawing figures do not limit the present invention to the specificembodiments disclosed and described herein. The drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the invention.

DETAILED DESCRIPTION

The following detailed description of embodiments of the inventionreferences the accompanying drawings. The embodiments are intended todescribe aspects of the invention in sufficient detail to enable thoseskilled in the art to practice the invention. Other embodiments can beutilized and changes can be made without departing from the scope of theclaims. The following detailed description is, therefore, not to betaken in a limiting sense. The scope of the present invention is definedonly by the appended claims, along with the full scope of equivalents towhich such claims are entitled.

In this description, references to “one embodiment”, “an embodiment”, or“embodiments” mean that the feature or features being referred to areincluded in at least one embodiment of the technology. Separatereferences to “one embodiment”, “an embodiment”, or “embodiments” inthis description do not necessarily refer to the same embodiment and arealso not mutually exclusive unless so stated and/or except as will bereadily apparent to those skilled in the art from the description. Forexample, a feature, structure, act, etc. described in one embodiment mayalso be included in other embodiments, but is not necessarily included.Thus, the present technology can include a variety of combinationsand/or integrations of the embodiments described herein.

The particular size and proportions of various aspects of the inventiondiscussed herein may vary from one embodiment of the invention toanother without departing from the spirit or scope of the invention.Therefore, any discussion herein of the dimensions and proportions areexemplary, and not limiting, in nature.

Turning now to the drawing figures, a seismic source apparatus 10constructed in accordance with embodiments of the invention isillustrated. The apparatus 10 broadly includes a seismic shock assembly12 and a mounting assembly 14 for securing the shock assembly 12 to avehicle 16. The vehicle 16 may be a pickup truck, van, sport utilityvehicle, tractor or other vehicle capable of carrying and enabling theapparatus 10 for its intended use. The apparatus 10 may be mounted on ahitch receiver, bumper, custom-made mounting system or other mountingmechanism capable of bearing the load associated with use of theapparatus 10. In some embodiments, the apparatus 10 is powered from thevehicle's power system such as, for example, a 12 volt DC vehicle powersystem or from a portable electric generator, such as a 120 volt ACgenerator.

The apparatus 10 may be used with a seismic surveying system 18including one or more seismic sensors 20. In some applications it may bedesirable to use the apparatus 10 with an array 22 of seismic sensors 20attached to a rope or cable and secured to the vehicle 16 such that asthe vehicle 16 pulls the sensor array 22 the array trails the vehicle ina linear configuration in a manner similar to that illustrated in FIG.2. The array 22 of seismic sensors 20 illustrated in FIG. 2, forexample, includes sixteen geophones attached to a rope or cable andspaced approximately one meter apart in a linear configuration extendingfrom the back of the vehicle 16 where the apparatus 10 may be mounted.The sensor array 22 may include horizontal axis and/or vertical axissensors. It may be advantageous to attach the array 22 to the vehicle 16such that it lies on a line corresponding to a center of the apparatus10, as explained below in greater detail.

With particular reference to FIGS. 3 and 4, the seismic shock assembly12 broadly includes a housing 24, one or more shock structures 26, 28, ahammer 30, 32 moveably disposed within each of the one or more shockstructures 26, 28, and a first actuator 34 for inducing movement in thehammers 30, 32. The illustrated embodiment includes two shock structures26, 28 removably fixed in opposing ends of the housing 24. In someembodiments, the shock structures 26, 28 are positioned and operatesymmetrically about a center of the housing 24. Such a configuration maybe advantageous, for example, where a seismic sensor array such as thearray 22 illustrated in FIG. 2 is aligned with a center of the apparatus10. In such a scenario, the seismic waves generated by the shockstructures 26, 28 are generated on both sides of, and symmetricallyabout, the sensor array 22. The shock structures 26, 28 may beidentically configured, therefore only the first shock structure 26 willbe described in detail herein with the understanding that the secondshock structure 28 is similarly configured.

The shock structure 26 includes a plurality of walls that define aninternal shock cavity 36 in which the hammer 30 operates. Morespecifically, a bottom wall 38 is part of both shock structures 26, 28and supports the actuator 34. A first end wall 40 proximate the actuator34 provides a strike surface 42 and therefore is configured to absorbrepeated blows from the hammer 30 without compromising structuralintegrity. The first end wall 40 may be, for example, two to four timesas thick as the other walls of the shock assembly 26. A second end wall44 distal the actuator 34 corresponds to an end of the cavity 36opposite the strike surface 42. A pair of side walls 46, 48 join the twoend walls 40, 44 such that the walls 40, 44, 46 and 48 define the shockcavity 36. The size and shape of the shock cavity 36 accommodatesmovement of the hammer 30 as it goes through a striking motion whereinit impacts the strike surface 42.

The end walls 40, 44 support a pair of rails 50 that extend from thefirst end wall 40 to the second end wall 44 and guide movement of thehammer 42 as it reciprocates within the shock cavity 36. The rails 50are removably secured to the shock structure 26 with collars 52enabling, for example, the hammer 30 to be removed from the shockstructure 26. A piston 54 extends through the first end wall 40 and iscoupled with both the actuator 34 and the hammer 30 and allows theactuator 34 to drive the hammer 30. In some embodiments, one or moresprings or similar elements (not shown) are interposed between thehammer 30 and the piston 54 to absorb the impact of the hammer 30against the strike surface 42 and protect the hammer 30 and the actuator34 from damage. In such embodiments, a selectively removable retainer orsimilar element (not shown) may be used to rigidly couple the hammer 30with the piston 54 to prevent the hammer 30 from moving relative to thepiston 54. The latter configuration may be desirable, for example, ifthe actuator 34 drives the hammer 30 in a reciprocating or “vibrating”motion without striking any surface of the shock structure 26.

As explained above, the bottom wall 38 may be a single, monolithicelement supporting both of the shock structures 26, 28. Alternatively,each of the shock structures 26, 28 may have a separate bottom wall toenable each shock structure 26, 28 to be removed from the housing 24separately from the other shock structure. Each of the end walls 40, 44and side walls 46, 48 may be fixedly or removably attached to the bottomwall 38. Thus, the shock structures 26, 28 may be removable from thehousing 24 as a single unit, or may be separately removable from thehousing 24. It may be desirable to remove one or both of the shockstructures 26, 28 from the housing, for example, for repair or forreplacement with another shock structure or structures presentingdifferent physical characteristics that generate seismic waves withdifferent characteristics.

The shock structure 26 may be constructed of virtually any rigidmaterial or combination of materials, such as steel or other metal. Theshock cavity 36 may be between about six inches and about eighteeninches in length, between about two inches and about eight inches inwidth, and between about one inch and about four inches in depth.

The hammer 30 is driven by the actuator 34 to impart kinetic energy intothe apparatus 10 and the ground, thus generating seismic waves in theground. The actuator 34 may impart a striking motion to the hammer 30wherein the hammer 30 impacts the strike surface 42 as a result of thestriking motion. Alternatively, the actuator 34 may impart vibratorymovement to the hammer 30 wherein the hammer 30 oscillates within thecavity 36 but does not strike any portion of the shock structure 26. Inthat scenario, acceleration and deceleration of the hammer 30 generatesseismic waves in the ground by transferring kinetic energy to thehousing 24 through the first actuator 34. Operation of the shockassembly 12 is described in greater detail below.

The hammer 30 includes a hammer body 56, a first insert 58 separablefrom the body 56 and a second insert 60 separable from the body 56. Asused herein, the inserts 58, 60 are “separable” from the body 56 if theyare configured to be separated from the body 56 and replaced withoutaltering or damaging the structure of the hammer 30. Each of the firstinsert 58 and the second insert 60 may be attached to the body 56 of thehammer 30 using bolts or similar fasteners. A metal strip 62 provides acontact for the seismic survey system 18 and may be secured to one ofthe inserts 58, 60.

The removable inserts 58, 60 allow the user to change the physicalcharacteristics of the hammer 30 and, consequently, the characteristicsof seismic waves generated by the hammer 30. Removing one or both of theinserts 58, 60, for example, reduces the weight of the hammer 30 whichaffects the acceleration and speed of the hammer 30 and the amount ofenergy transferred from the hammer 30 to the shock structure 26 throughthe strike surface 42. Alternatively, one or both of the inserts 58, 60may be removed and replaced with different inserts bearing differentphysical properties, such as weight, that generate seismic waves withdifferent properties.

The hammer 30 includes through-holes 64 that engage the rails 50 as thehammer moves within the cavity 36. The hammer 30 is configured to beremovable from the shock structure 26 for repairs or replacement. Thehammer 30 is removed from the shock structure 26 by removing the collars52 to allow the rails 50 to be disengaged from the hammer 30 anddisengaging the hammer 30 from the piston 54. Disengaging the hammer 30from the piston 54 may involve, for example, removing a bolt or otherfastener as well as one or more springs and/or retainers, describedabove. A hammer is mounted in the shock structure 26 by reversing thisprocess.

It will be appreciated that the configuration of the shock structure 26and the hammer 30 allows users to modify the physical characteristics ofthe hammer 30 and, thus, characteristics of seismic waves generated bythe shock structure 26. Physical characteristics of the hammer 30 may bealtered by removing or replacing one or both of the inserts 58, 60, andthe hammer 30 may be replaced in its entirety by another hammerpresenting different characteristics. Testing has determined, forexample, that in some embodiments hammers made of denser (heavier)materials tend to generate seismic waves with lower frequency contentand hammers made of less dense (lighter) materials generate seismicwaves of higher frequencies. Hammers made of iron, for example,generated seismic waves with lower frequency content while hammers madeof aluminum generated seismic waves with higher frequency content. Thus,the shock structures 26, 28 and/or the hammers 30, 32 may be selectedfrom various available shock structures and hammers to meet the needs ofa particular use or application.

As mentioned above, the hammer 30 may include a metal strip 62 providinga contact for the seismic survey system 18. More particularly, the strip62 may communicate a signal to the seismic survey system 18 each timethe hammer 30 strikes the strike surface 42 of the shock structure 26.The survey system 18 may use the signal, for example, for timingoperation of the sensors 20, for timing operation of an imagingapparatus, or both. While the metal strip 62 provides a relativelysimple and inexpensive trigger for the survey system 18, other triggerdevices and methods are within the ambit of the present invention. Byway of example, optical or inductive sensors may be used in place of orin addition to the metal strip 62.

The hammer 30 may be virtually any size, shape and configuration and maybe constructed of any of various materials. In some embodiments, thehammer 30 weighs between about two pounds and about ten pounds and mayparticularly weigh, for example, about four pounds, about five pounds orabout six pounds.

The mounting assembly 14 secures the apparatus 10 to the vehicle 16 andmoves the seismic shock assembly 12 between deployed and retractedpositions. The mounting assembly 14 broadly includes mounting hardware66, a pair of extension mechanisms 68, 70 for moving the shock assembly12 relative to the mounting assembly 14, a pair of arms 72, 74 forstabilizing and guiding movement of the shock assembly 12 relative tothe mounting assembly 14, and a second actuator 76 for driving theextension mechanisms 68, 70. In the illustrated embodiment, the mountinghardware 66 includes a metal tube that engages a hitch receiver orsimilar mechanism on the vehicle 16 and locks in place in the hitchreceiver in a manner similar to a standard tow hitch.

Each of the extension mechanisms 68, 70 includes pistons or otherelements operable to extend and retract when driven by the secondactuator 76. In some embodiments the second actuator 76 is a pneumaticactuator using compressed air to drive pneumatic pistons in theextension mechanisms 68, 70. The mounting assembly 14 may applysubstantial downward pressure, such as between two hundred and onethousand pounds, on the shock assembly 12 during use. Therefore, themounting hardware 66, extension mechanisms 68, 70 and other componentsof the mounting assembly 14 are sufficiently strong to endure repeatedapplication of such high pressure. The arms 72, 74 guide movement of theshock assembly 12 while preventing lateral movement of the shockassembly 12 relative to the apparatus 10, and may include springs forbiasing the shock assembly toward the retracted position.

When the shock assembly 12 is in the deployed position, as illustratedin FIG. 1, it engages the ground such that the walls of the shockstructures 26, 28 are normal to the surface of the ground. Because thestrike surface 42 is normal to the surface of the ground, the impact ofthe hammer 30 against the strike surface 42 generates seismic shearwaves in the ground. When the shock assembly 12 is in the retractedposition it is separated from the surface of the ground by a distanceand the apparatus 10 is supported entirely by the vehicle 16. The shockassembly 12 may be placed in the retracted position when travelling to atesting location, for example, or when travelling between seismicsurveys.

The mounting assembly 14 may include any of various types of actuators,including electric, hydraulic or pneumatic actuators for drivingoperation of the extension mechanisms 68, 70.

A controller 80 manages operation of the mounting assembly 14 and theshock assembly 12 and may be housed in a console 82 that includes one ormore buttons, switches, displays or other user interface 84 elements. Inparticular, the controller 80 directs operation of the first actuator 34and the second actuator 76. The controller 80 may communicate with themounting assembly 14 and the shock assembly 12 remotely via a wired orwireless connection. In some embodiments, a user may hold the console 82and operate the apparatus 10 from a remote position, such as inside thevehicle 16.

The controller 80 may include a digital integrated circuit such as, forexample, a general use computer processor or a programmable logic deviceconfigured for operation with the circuit receiver. The controller 80may further include memory elements for storing data, instructions, orboth used by the integrated circuit to implement the functionality ofthe controller 80. The functionality of the controller 80 may beimplemented by components housed in the console 82 and/or in anothersection of the apparatus 10, such as in the shock assembly 12, themounting assembly 14, or both.

In operation, the apparatus 10 is secured to the vehicle 16 by, forexample, coupling the mounting hardware 66 to a hitch receiver. Theapparatus 10 may be connected to the vehicle's 16 power system byconnecting a power connector of the apparatus 10 (not shown) to acorresponding connector on the vehicle 16 that is located on or near thevehicle's bumper and thus is proximate the apparatus 10 when theapparatus 10 is attached to the vehicle 16.

The seismic sensor array 22 may also be secured to the vehicle 16, suchas in the manner illustrated in FIG. 2. With the vehicle 16 and thesensor array 22 in position, the mounting assembly 14 is actuated todeploy the shock assembly 12 in preparation for performing a seismicsurvey. Deploying the shock assembly 12 may involve directing the secondactuator 76 to drive the extension mechanisms 68, 70 downward such thatthe shock assembly 12 engages the ground. The second actuator 76 appliessufficient downward pressure on the shock assembly 12 such that theenergy from the hammer strikes is transferred to the ground. Thus, theshock assembly 12 must generally be held against the ground withsufficient force that the shock assembly 12 does not move relative tothe ground when the hammers 30, 32 impact the shock structures 26, 28.As explained above, downward force of one thousand pounds or more on theshock assembly 12 may be required to effectively transfer energy fromthe shock assembly 12 to the ground.

When the shock assembly 12 is deployed the user directs the shockassembly 12 to generate one or more seismic waves by controlling thefirst actuator 34 to move the hammers 30, 32 in a striking motionwherein the hammers 30, 32 impact strike surfaces of the shockstructures 26, 28. The first actuator 34 may drive one of the hammers30, 32 to generate a single strike, may first drive one of the hammers30, 32 to generate a strike and then drive the other of the hammers 30,32 to generate a strike, or may cause one or both of the hammers 30, 32to repeatedly strike the shock structure or shock structures.Alternatively, the controller 80 may direct the first actuator 34 tomove one or both of the hammers 30, 32 in an oscillating motion withoutstriking any surfaces of the shock assembly 12. In that scenario, theacceleration and deceleration of one or both of the hammers 30, 32transfers energy to the shock assembly 12 and the ground, as explainedabove. As used herein, a hammer oscillates without striking a surface ifit does not impact any surface with sufficient force to cause adetectable seismic disturbance.

While the seismic waves associated with hammer strikes include multiplefrequency components (see, for example, FIGS. 7-9), seismic wavesgenerated by hammer oscillations generally include a single frequencycomponent corresponding to the hammer oscillation frequency.Furthermore, the seismic waves generated by hammer oscillations arecontinuous and can be manipulated by adjusting the oscillation frequencyof the hammers. It may be desirable, for example, to conduct a seismicsurvey across a range of frequencies beginning with a low frequency andgradually increasing to a high frequency, or vice-versa. By way ofexample, the apparatus 10 may be operable to oscillate the hammers 30,32 at a frequency of between 0 and 150 Hz to generate seismic waves atany frequency within that range.

In some implementations, the hammers 30, 32 are operated to generateseismic waves that are identical in magnitude and frequency but are 180°out of phase. The seismic survey system 18 may use the two signals toincrease the integrity of the survey by combining the signals toidentify and eliminate noise.

The controller 80 may adjust operation of the first actuator 34 to alterthe characteristics of the striking motion of the hammers 30, 32 and tochange the characteristics of the seismic waves resulting from themovement and/or impact of the hammers 30, 32. More particularly, thecontroller 80 may adjust operation of the actuator 34 to increase ordecrease the acceleration or speed of the hammers 30, 32, as well as thefrequency of the hammer strikes. By way of example, the rate ofacceleration of the hammers 30, 32 may be within the range of from about10 m/sec² to about 50 m/sec², and may particularly be about 20 m/sec²,about 25 m/sec², about 30 m/sec², about 35 m/sec² or about 40 m/sec².The maximum speed of the hammers 30, 32 and/or the speed at which thehammers 30, 32 strike a surface of the shock structures 26, 28 may befrom about 1 m/sec to about 5 m/sec and may particularly be about 2m/sec, about 3 m/sec or about 4 m/sec.

The seismic survey system 18 includes a controller 86 that collects datafrom the sensors 20 and from the metal strips 62, 62′, processes thedata, and generates information about the subterranean area undersurvey. The generated information may be presented to a user via a userinterface 88 in the form of an image, paper trace or otherhuman-readable form of expression.

FIGS. 7-9 illustrate an advantage of the apparatus 10 over existingseismic survey systems. FIG. 7 is a graph of the frequency content of anexemplary seismic shear wave generated manually using a ten pound sledgehammer, and FIG. 9 is a graph of the frequency content of an exemplaryseismic shear wave generated manually using a two pound sledge hammer.FIG. 8 is a graph of the frequency content of an exemplary seismic shearwave generated by the apparatus using a five pound hammer. Asillustrated, the apparatus generates a seismic wave including strongfrequency components across a greater range of frequencies than eitherthe two pound sledge hammer or the ten pound sledge hammer. The strongerfrequency profile of the waves generated by the apparatus 10 isadvantageous because seismic waves encompassing a broader range ofseismic frequencies are capable of reacting to a broader range ofsubterranean discontinuities, thereby enabling the system 18 togenerated more detailed information about the area under survey.

It will also be appreciated that the apparatus 10 improves upon theperformance of a manual sledge hammer by generating uniform andrepeatable seismic waves symmetrically about an linear sensor array,such as the array 22. For any given deployment of the shock assembly 12,for example, each strike of the hammer 30 (and of the hammer 32) willgenerate substantially identical seismic waves. By way of example, thecontroller 86 may use multiple data sets based on identical seismicwaves to identify and ignore statistically outlying characteristics ofthe data.

The apparatus 10 is well suited for use with the system 18 to detectrelatively small subterranean voids at relatively small depths. Theapparatus 10 could be used, for example, to detect tunnels created forhuman passage, which may be less than five feet across and less thanthree hundred feet deep.

Although the invention has been described with reference to thepreferred embodiment illustrated in the attached drawing figures, it isnoted that equivalents may be employed and substitutions made hereinwithout departing from the scope of the invention as recited in theclaims. By way of example, the shock cavity may be defined by structuralelements integrally formed in the shock assembly housing rather than byremovable shock structures.

Having thus described the preferred embodiment of the invention, what isclaimed as new and desired to be protected by Letters Patent includesthe following:
 1. An apparatus for generating seismic waves, theapparatus comprising: a housing; a first shock structure removablydisposed within the housing and defining a first shock cavity; a firsthammer movably disposed within the first shock structure; a second shockstructure removably disposed within the housing and defining a secondshock cavity; a second hammer movably disposed within the second shockstructure; a first pair of rails extending along a direction of movementof the first hammer within the first shock structure and beingsymmetrically offset from a center of the first hammer for guidingmovement of the first hammer within the first shock structure, the firsthammer being slidably mounted on the first pair of rails; a second pairof rails extending along a direction of movement of the second hammerwithin the second shock structure and being symmetrically offset from acenter of the second hammer for guiding movement of the second hammerwithin the second shock structure, the second hammer being slidablymounted on the second pair of rails; and an actuator for inducing astriking motion in the hammers wherein the hammers each impact a strikesurface as part of the striking motion, the actuator being selectivelyadjustable to change characteristics of the striking motion andcharacteristics of seismic waves generated by the impact.
 2. Theapparatus of claim 1, the actuator configured to move the hammers in areciprocating motion along a linear path that is normal to the strikesurfaces.
 3. The apparatus of claim 1, the actuator being operable tomove the hammers in a reciprocating motion along a linear path andwithout striking any portion of the housing.
 4. The apparatus of claim1, the actuator including a linear electric motor.
 5. The apparatus ofclaim 1, wherein the shock structures are removably disposed within thehousing, the strike surface being an interior surface of the shockcavity and the hammer being disposed within the shock cavity.
 6. Theapparatus of claim 5, the hammers being removably disposed within therespective shock cavity.
 7. The apparatus of claim 1, the housing beingconfigured such that when the housing is placed on contact with a groundsurface, the impact of the hammers against the strike surfaces generatesshear seismic waves in the ground.
 8. The apparatus of claim 1, furthercomprising a mounting assembly configured to secure the apparatus to avehicle and to move the apparatus between a deployed position and aretracted position, wherein when the apparatus is in the deployedposition the housing engages a ground surface and the strike surfacesare normal to the ground surface.
 9. The apparatus of claim 8, themounting assembly including a pneumatic actuator for moving theapparatus between the deployed position and the retracted position. 10.The apparatus of claim 1, the hammers each including a first portion anda second portion, the second portion being separable from the firstportion.
 11. The apparatus of claim 1, the hammers each weighing betweentwo pounds and ten pounds and including a first portion made of a firstmaterial and a second portion made of a second material, the secondportion being separable from the first portion.
 12. The apparatus ofclaim 11, the hammers each presenting a rectangular shape, weighingbetween four pounds and six pounds, and including an aluminum portionand at least one steal portion separably coupled with the aluminumportion.
 13. An apparatus for generating seismic waves, the apparatuscomprising: a housing; a first shock structure removably disposed withinthe housing and defining a first shock cavity; a first hammer movablydisposed within the first shock structure; a second shock structureremovably disposed within the housing and defining a second shockcavity; a second hammer movably disposed within the second shockstructure; and an actuator operable to move the hammers and cause eachof the hammers to strike an internal surface with a striking motion, theactuator being selectively adjustable to modify characteristics of thestriking motion of each of the hammers, including hammer speed andacceleration.
 14. The apparatus of claim 13, the first shock structureand the second shock structure being positioned symmetrically onopposite sides of a center of the housing.
 15. The apparatus of claim13, further comprising a mounting assembly configured to secure theapparatus to a vehicle and to move the apparatus between a deployedposition and a retracted position, wherein when the apparatus is in thedeployed position the housing engages a ground surface and each of thestrike surfaces is normal to the ground surface.
 16. The apparatus ofclaim 15, the mounting assembly including a pneumatic actuator formoving the apparatus between the deployed position and the retractedposition.
 17. The apparatus of claim 13, the first hammer and the secondhammer each including a first portion and a second portion, the secondportion being separable from the first portion.
 18. An apparatus forgenerating seismic waves, the apparatus comprising: a housing; a firstshock structure removably disposed within the housing and defining afirst shock cavity; a first hammer movably disposed within the firstshock structure; a second shock structure removably disposed within thehousing and defining a second shock cavity; a second hammer movablydisposed within the second shock structure; an actuator for inducing astriking motion in the hammers wherein the hammers each impact a strikesurface as part of the striking motion, the actuator being selectivelyadjustable to change characteristics of the striking motion andcharacteristics of seismic waves generated by the impact; and a set ofseismic sensors attached to a straightenable line and configured to bespaced apart from each other and aligned in a linear configuration whenthe line is straightened.
 19. The apparatus of claim 18, wherein theapparatus is configured to be mounted on a hitch receiver of a vehicle,and the straightenable line is configured to be straightened behind thevehicle such that the seismic sensors become spaced apart from eachother and aligned in the linear configuration when the vehicle is drivenforward.